Optimize subtype checking by avoiding a nested function

Mypyc isn't good at compiling nested functions, and this one was
in one of the hottest code paths in all of mypy. The nested
function wasn't even used all the time, but mypyc will still
construct a closure object every time.

This adds some code duplication, but it's well worth it. Amazingly,
this speeds up self-check by about 10%!

This addresses some of the slowdown introduced in #13303. #14324
addresses another related slowdown.

mypy/subtypes.py

@@ -289,18 +289,20 @@ def _is_subtype(
         # ErasedType as we do for non-proper subtyping.
         return True
 
-    def check_item(left: Type, right: Type, subtype_context: SubtypeContext) -> bool:
-        if proper_subtype:
-            return is_proper_subtype(left, right, subtype_context=subtype_context)
-        return is_subtype(left, right, subtype_context=subtype_context)
-
     if isinstance(right, UnionType) and not isinstance(left, UnionType):
         # Normally, when 'left' is not itself a union, the only way
         # 'left' can be a subtype of the union 'right' is if it is a
         # subtype of one of the items making up the union.
-        is_subtype_of_item = any(
-            check_item(orig_left, item, subtype_context) for item in right.items
-        )
+        if proper_subtype:
+            is_subtype_of_item = any(
+                is_proper_subtype(orig_left, item, subtype_context=subtype_context)
+                for item in right.items
+            )
+        else:
+            is_subtype_of_item = any(
+                is_subtype(orig_left, item, subtype_context=subtype_context)
+                for item in right.items
+            )
         # Recombine rhs literal types, to make an enum type a subtype
         # of a union of all enum items as literal types. Only do it if
         # the previous check didn't succeed, since recombining can be
@@ -312,9 +314,16 @@ def check_item(left: Type, right: Type, subtype_context: SubtypeContext) -> bool
             and (left.type.is_enum or left.type.fullname == "builtins.bool")
         ):
             right = UnionType(mypy.typeops.try_contracting_literals_in_union(right.items))
-            is_subtype_of_item = any(
-                check_item(orig_left, item, subtype_context) for item in right.items
-            )
+            if proper_subtype:
+                is_subtype_of_item = any(
+                    is_proper_subtype(orig_left, item, subtype_context=subtype_context)
+                    for item in right.items
+                )
+            else:
+                is_subtype_of_item = any(
+                    is_subtype(orig_left, item, subtype_context=subtype_context)
+                    for item in right.items
+                )
         # However, if 'left' is a type variable T, T might also have
         # an upper bound which is itself a union. This case will be
         # handled below by the SubtypeVisitor. We have to check both